Pathogenic bacteria are a substantial cause of sickness and death in both humans and animals. Prominent among these is Staphylococcus aureus (S. aureus; SA) which is the leading cause of bacterial infections in humans worldwide. S. aureus can cause a range of illnesses, from minor skin infections to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome (TSS), bacteremia, and sepsis. Its incidence ranges from skin, soft tissue, respiratory, bone, joint, endovascular to wound infections. It is still one of the five most common causes of nosocomial infections and is often the cause of postsurgical wound infections. Each year, some 500,000 patients in American hospitals contract a staphylococcal infection.
Over the last several decades infection with S. aureus is becoming increasingly difficult to treat largely due to the emergence of methicillin-resistant S. aureus (MRSA) that is resistant to all known beta-lactam antibiotics (Boucher, H. W. et al. Bad bugs, no drugs: no ESKAPE! An update from the Infectious Diseases Society of America. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America 48, 1-12 (2009)). The circumstances are so acute, that by 2005, infection with MRSA was reported to be the leading cause of death due to a single infectious agent—responsible for over 15,000 deaths in the United States (DeLeo, F. R. & Chambers, H. F. Reemergence of antibiotic-resistant Staphylococcus aureus in the genomics era. The Journal of Clinical Investigation 119, 2464-2474 (2009)). Vancomycin, linezolid and daptomycin have become the antibiotics of choice for treating invasive MRSA infections (Boucher, H., Miller, L. G. & Razonable, R. R. Serious infections caused by methicillin-resistant Staphylococcus aureus. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America 51 Suppl 2, S183-197 (2010)). However, reduced susceptibility to vancomycin and cross-resistance to linezolid and daptomycin have also been reported in MRSA clinical strains (Nannini, E., Murray, B. E. & Arias, C. A. (2010) “Resistance or decreased susceptibility to glycopeptides, daptomycin, and linezolid in methicillin-resistant Staphylococcus aureus.” Current opinion in pharmacology 10, 516-521). Over time, the vancomycin dose necessary to overcome resistance has crept upward to levels where nephrotoxicity occurs. Thus, mortality and morbidity from invasive MRSA infections remains high despite these antibiotics.
Although SA is generally thought to be an extracellular pathogen, investigations going back at least 50 years have revealed its ability to infect and survive in various types of host cells, both professional phagocytes and non-phagocytic cells (Gresham, H. D. et al. Survival of Staphylococcus aureus inside neutrophils contributes to infection. J Immunol 164, 3713-3722 (2000); Anwar, S., Prince, L. R., Foster, S. J., Whyte, M. K. & Sabroe, I. The rise and rise of Staphylococcus aureus: laughing in the face of granulocytes. Clinical and Experimental Immunology 157, 216-224 (2009); Fraunholz, M. & Sinha, B. Intracellular staphylococcus aureus: Live-in and let die. Frontiers in cellular and infection microbiology 2, 43 (2012); Garzoni, C. & Kelley, W. L. Return of the Trojan horse: intracellular phenotype switching and immune evasion by Staphylococcus aureus. EMBO molecular medicine 3, 115-117 (2011)). This facultative intracellular persistence enables host immune evasion, long-term colonization of the host, maintenance of a chronically infected state, and is likely a cause for clinical failures of, and relapses after, conventional antibiotic therapy. Furthermore, exposure of intracellular bacteria to suboptimal antibiotic concentrations may encourage the emergence of antibiotic resistant strains, thus making this clinical problem more acute. Consistent with these observations, treatment of patients with invasive MRSA infections such as bacteremia or endocarditis with vancomycin or daptomycin was associated with failure rates greater than 50% (Kullar, R., Davis, S. L., Levine, D. P. & Rybak, M. J. Impact of vancomycin exposure on outcomes in patients with methicillin-resistant Staphylococcus aureus bacteremia: support for consensus guidelines suggested targets. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America 52, 975-981 (2011); Fowler, V. G., Jr. et al. Daptomycin versus standard therapy for bacteremia and endocarditis caused by Staphylococcus aureus. The New England journal of medicine 355, 653-665 (2006); Yoon, Y. K., Kim, J. Y., Park, D. W., Sohn, J. W. & Kim, M. J. Predictors of persistent methicillin-resistant Staphylococcus aureus bacteraemia in patients treated with vancomycin. The Journal of antimicrobial chemotherapy 65, 1015-1018 (2010)). Therefore, a more successful anti-staphylococcal therapy should include the elimination of intracellular bacteria.
Most of today's antibacterials are semisynthetic modifications of various natural compounds. These include, for example, the beta-lactam antibacterials, which include the penicillins (produced by fungi in the genus Penicillium), the cephalosporins, and the carbapenems. Antimicrobial compounds that are still isolated from living organisms include the aminoglycosides, whereas other antibacterials—for example, the sulfonamides, the quinolones, and the oxazolidinones, are produced solely by chemical synthesis. In accordance with this, many antibacterial compounds are classified on the basis of chemical/biosynthetic origin into natural, semisynthetic, and synthetic. Another classification system is based on biological activity; in this classification, antibacterials are divided into two broad groups according to their biological effect on microorganisms: bactericidal agents kill bacteria, and bacteriostatic agents slow down or stall bacterial growth.
Ansamycins are a class of antibiotics, including rifamycin, rifampin, rifampicin, rifabutin, rifapentine, rifalazil, ABI-1657, and analogs thereof, that inhibit bacterial RNA polymerase and have exceptional potency against gram-positive and selective gram-negative bacteria (Rothstein, D. M., et al (2003) Expert Opin. Invest. Drugs 12(2):255-271; U.S. Pat. Nos. 7,342,011; 7,271,165).
Immunotherapies have been reported for preventing and treating S. aureus (including MRSA) infections. US2011/0262477 concerns uses of bacterial adhesion proteins Eap, Emp and AdsA as vaccines to stimulate immune response against MRSA. WO2000/071585 describes isolated monoclonal antibodies reactive to specific S. aureus strain isolates. US2011/0059085 suggests an Ab-based strategy utilizing IgM Abs specific for one or more SA capsular antigens, although no actual antibodies were described.
Teichoic acids (TA) are bacterial polysaccharides found within the cell wall of Gram-positive bacteria including SA. Wall teichoic acids (WTA) are those covalently linked to the peptidoglycan (PDG) layer of the cell wall; whereas lipoteichoic acids (LTA) are those covalently linked to the lipids of the cytoplasmic membrane. Xia et al. (2010) Intl. J. Med. Microbiol. 300:148-54. These glycopolymers play crucial roles in bacterial survival under disadvantageous conditions and in other basic cellular processes. The known WTA structures vary widely between bacterial species. S aureus TAs are composed of repetitive polyol phosphate subunits such as ribitol phosphate or glycerol phosphate. Given their structural diversity and variability, WTAs are considered attractive targets for antibodies and as vaccines, ibid.
Antibody-drug conjugates (ADC), also known as immunoconjugates, are targeted chemotherapeutic molecules which combine ideal properties of both antibodies and cytotoxic drugs by targeting potent cytotoxic drugs to antigen-expressing tumor cells (Teicher, B. A. (2009) Curr. Cancer Drug Targets 9:982-1004), thereby enhancing the therapeutic index by maximizing efficacy and minimizing off-target toxicity (Carter, P. J. and Senter P. D. (2008) The Cancer J. 14(3):154-169; Chari, R. V. (2008) Acc. Chem. Res. 41:98-107. ADC comprise a targeting antibody covalently attached through a linker unit to a cytotoxic drug moiety. Immunoconjugates allow for the targeted delivery of a drug moiety to a tumor, and intracellular accumulation therein, where systemic administration of unconjugated drugs may result in unacceptable levels of toxicity to normal cells as well as the tumor cells sought to be eliminated (Polakis P. (2005) Curr. Opin. Pharmacol. 5:382-387). Effective ADC development for a given target antigen depends on optimization of parameters such as target antigen expression levels, tumor accessibility (Kovtun, Y. V. and Goldmacher V. S. (2007) Cancer Lett. 255:232-240), antibody selection (U.S. Pat. No. 7,964,566), linker stability (Erickson et al (2006) Cancer Res. 66(8):4426-4433; Doronina et al (2006) Bioconjugate Chem. 17:114-124; Alley et al (2008) Bioconjugate Chem. 19:759-765), cytotoxic drug mechanism of action and potency, drug loading (Hamblett et al (2004) Clin. Cancer Res. 10:7063-7070) and mode of linker-drug conjugation to the antibody (Lyon, R. et al (2012) Methods in Enzym. 502:123-138; Xie et al (2006) Expert. Opin. Biol. Ther. 6(3):281-291; Kovtun et al (2006) Cancer Res. 66(6):3214-3121; Law et al (2006) Cancer Res. 66(4):2328-2337; Wu et al (2005) Nature Biotech. 23(9):1137-1145; Lambert J. (2005) Current Opin. in Pharmacol. 5:543-549; Hamann P. (2005) Expert Opin. Ther. Patents 15(9):1087-1103; Payne, G. (2003) Cancer Cell 3:207-212; Trail et al (2003) Cancer Immunol. Immunother. 52:328-337; Syrigos and Epenetos (1999) Anticancer Res. 19:605-614).
The concept of ADC in cancer therapy has also been expanded into antibacterial therapy, in this case the drug portion is an antibiotic, resulting in antibody-antibiotic conjugate (AAC). U.S. Pat. No. 5,545,721 and U.S. Pat. No. 6,660,267 describe synthesis of a non-specific immunoglobulin-antibiotic conjugate that binds to the surface of target bacteria via the antibiotic, and uses thereof for treating sepsis. U.S. Pat. No. 7,569,677 and related patents suggest prophetically antibiotic-conjugated antibodies that have an antigen-binding portion specific for a bacterial antigen (such as SA capsular polysaccharide), but lack a constant region that reacts with a bacterial Fc-binding protein (e.g., staphylococcal protein A).